Phosphorus-based flame retardants are receiving more and more attention in the market because of their good flame retardant performance, low smoke, low toxicity, low dosage, high efficiency and wide application. Phosphorus-based flame retardants can be divided into two categories: reactive and additive. Reactive products are bonded to polymer materials through chemical reactions to achieve long-lasting flame retardancy; additive phosphorus-based flame retardants are physically mixed with polymer materials and then processed into polymer composites with flame retardant functions. Additive flame retardants are easy to use and occupy a dominant position in the field of PC flame retardant applications. Phosphorus-based flame retardants can be divided into two categories: inorganic phosphorus-based flame retardants and organic phosphorus-based flame retardants. At present, organic phosphorus flame retardants are more widely used in the field of flame retardant organic materials.

The flame retardant mechanism of organophosphorus flame retardants can be explained in four aspects: 1. The flame retardant decomposes by combustion and catalyzes the dehydration of the polymer to form a carbon layer; 2. PO-radicals are produced, while polymers are decomposed to form a small amount of hydrogen radicals (H-) and hydroxyl radicals (HO-) during combustion, and PO-radicals produced by the decomposition of phosphorus-based flame retardants can capture H- and HO- to form HPO, thus preventing or slowing down the combustion chain reaction and enhancing the flame retardant effect; 4. Reduce the concentration of combustible gases, thereby delaying the spread of flame.

 I. Phosphate ester flame retardants.

Most phosphate ester flame retardants are additive flame retardants, which have the dual functions of flame retardant and plasticizer. The phosphorus-based flame retardants commonly used in the market are mainly triphenyl phosphate (TPP), resorcinol bis(diphenyl phosphate) (RDP) and bisphenol A bis(diphenyl phosphate) (BDP). The three are slightly different in flame retardant mechanism, among which TPP is volatile at high temperature and can only play the role of flame retardant in the gas phase, while RDP and BDP can play the role of flame retardant in both gas phase and solid phase. The test shows that when the flame retardant addition amount is 10% (wt), the 1.6mm thickness PC sample UL-94 test can reach V-0 flame retardant level. The poor hydrolysis resistance of phosphate ester flame retardants is currently the main defect in the application of organic materials.

Second, phosphonate ester flame retardants.

The current research on phosphonate flame retardants mainly focuses on nitrogen-containing phosphonate flame retardants and reactive phosphonate flame retardants. Phosphonate ester molecular stability, good water resistance and solvent resistance, with the dual role of flame retardant and plasticizing, can be used in the flame retardant treatment of a variety of polymer materials. Phosphonate flame retardants have good thermal stability, moderate price, good flame retardant effect, low impact on the mechanical properties of materials, and good resistance to hydrolysis, and have good prospects for application in the field of flame retardant.

Third, phosphorus heterophilic flame retardants.

DOPO molecule has excellent stability, good hydrolysis resistance, and also has many advantages such as halogen-free, non-toxic and flame retardant, etc. Compared with phosphate ester flame retardants, its impact on the heat resistance and mechanical properties of the base resin is smaller, and is now a hot spot for flame retardant research for organic materials. It is generally believed that DOPO exhibits a flame retardant mechanism in the gas phase, and when DOPO is exposed to a high temperature environment, it releases phosphorus-containing radicals and phenoxy radicals to quench the combustion chain reaction in the gas phase to achieve flame retardant effects. DOPO molecules with phosphorus-containing skeleton are good intermediates for flame retardant synthesis.

IV. Phosphonitrile flame retardants.

Phosphonitrile molecules are rich in phosphorus and nitrogen elements, which have a good synergistic flame retardant effect and are good flame retardant materials. Phosphonitrile molecules can generally be divided into two categories: cyclic phosphonitrile and linear phosphonitrile.  ), which can terminate the free radical chain reaction and dilute the concentration of flammable gases, slowing down the further combustion of organic materials and showing the gas-phase flame retardant effect.

Hexachlorocyclotriphosphonitrile molecules have many reaction sites, and different types of cyclotriphosphonitrile flame retardants can be prepared by chemical modification, which are ideal intermediates for flame retardant synthesis. Phosphonitrile flame retardants have the advantages of high flame retardant efficiency, low environmental pollution and good dispersion performance in PC materials, which have certain application prospects in the field of flame retardant PC, but the production cost of such flame retardants is high.

 V. Inorganic phosphorus flame retardants.

 At present, the application of inorganic phosphorus flame retardants in PC flame retardant is relatively small. The flame retardant efficiency of inorganic phosphorus flame retardants is low, and 40% to 60% (wt) of flame retardant needs to be added to make the PC flame retardant products reach V-0 flame retardant grade in UL-94 vertical combustion test. However, increasing the amount of additive has a great impact on the mechanical properties of the material, and the strength, elastic modulus, compatibility and stability of the material will be reduced, so it is necessary to improve the combination performance of flame retardant and PC by modifying the additive, so as to improve the thermal stability of the product.

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